A large amount of technological interest has been focused on rare earth-iron-boron alloys (e.g., 26.7 weight % Nd-72.3 weight % Fe-1.0 weight %-B) as a result of their promising magnetic properties for permanent magnet applications attributable to the magnetically hard Nd.sub.2 Fe.sub.14 B phase. Commercial permanent magnets of these alloys having anisotropic, aligned structure exhibit high potential energy products (i.e., BHmax) of 40-48 MGOe while those having an isotropic, non-aligned structure exhibit potential energy products of 5-10 MGOe. Such energy product levels are much higher than those exhibited by Sm-Co alloys (e.g., SmCo.sub.5 and Sm.sub.2 Co.sub.17) previously regarded as having optimum magnetic properties. The rare earth-iron-boron alloys are also advantageous over the SmCo alloys in that the rare earth (e.g., Nd) and Fe are much more abundant and economical than Sm and Co. As a result, rare earth-iron-boron permanent magnets are used in a wide variety of applications including, but not limited to, audio loud speakers, electric motors, generators, meters, scientific instruments and the like.
Two different approaches are currently in use to produce isotropic permanent magnets from rare earth-iron-boron alloys (e.g., Nd--Fe--B). One approach involves rapidly solidifying the Nd--Fe--B alloy by melt spinning to produce a near-amorphorous, fine grained ribbon material, mechanically comminuting the ribbon to form flake particulates, and then vacuum hot pressing the flakes in a die cavity to consolidate the material. This approach suffers from numerous disadvantages such as microstructural inhomogeneities induced by non-uniform quenching, contamination and non-ideal particle shape (e.g., thin platelets) for further magnet fabrication operations. The vacuum hot pressing operation typically requires at least brief exposure to a partial liquidification (melting) temperature to enhance interparticle bonding.
The second approach involves mechanical comminution of a chill cast ingot and "powder metallurgy" consolidation of the resulting fine comminuted alloy powder wherein the fine comminuted powder is pressed and sintered using liquid phase sintering and long time anneals (e.g., total anneal times up to 25 hours) to consolidate the powder. This latter approach has traditionally been used to fabricate SmCo, ferrite and other types of magnets. This latter approach suffers from numerous disadvantages such as explosibility hazards, contamination, microstructural inhomogeneities, excessive grain growth and, as mentioned, long processing times.
Both of the aforementioned fabrication approaches thus are disadvantageous in that they involve difficult-to-process, irregular-shape alloy particulates using complex, time consuming and high cost particulate processing and heat treatment techniques. The magnets fabricated in these ways from such alloy particulates are prone to inhomogeneities in microstructure and composition that can adversely affect the desired isotropic magnetic properties of the magnet.
It is an object of the present invention to provide a method of making isotropic permanent magnets from rare earth-transition metal (e.g., iron) alloy particles in a manner that overcomes the disadvantages of the fabrication approaches described hereinabove.
It is another object of the present invention to provide a method of making isotropic permanent magnets from rare earth-transition metal alloy particles wherein processing times and steps are reduced and simplified and wherein the excessive heat treatment requirements of the fabrication approaches described hereinabove are eliminated so as to reduce the cost of producing the isotropic magnets.
It is still another object of the present invention to provide a method of making isotropic permanent magnets from rare earth-transition metal alloy particles wherein the microstructures and compositions of the fabricated magnets exhibit improved homogeneity as compared to isotropic permanent magnets fabricated by the approaches described hereinabove.
It is still a further object of the present invention to provide a method of making isotropic permanent magnets from rare earth-transition metal alloy particles wherein the magnets have dramatically improved mechanical strength.